Alkaline divalent silver oxide cells have been known for a considerable period of time. However, a requirement for miniature, high energy capacity cells of the button cell variety--where there is a high power output capability and high energy storage per unit volume--has recently arisen particularly in view of the wide acceptance of electrically powered watches and the like. During its shelf life or its operating life, it is important that these cells demonstrate dimensional stability; and to do so, the rate of oxygen evolution within the cell must not be greater than the sum of the rates of oxygen diffusion to the anode of the cell or oxygen reaction with the anode, and oxygen escape from the cell.
Because of their high power output and energy storage per unit volume, divalent silver oxide cells are particularly attractive. However, divalent silver oxide is generally considered to be unstable in the presence of an aqueous electrolyte, such as any of the alkaline electrolytes generally used in commercial silver oxide cells. It has particularly been desirable to improve the stability of divalent silver oxide cells so as to take advantage of the characteristics stated above. One approach has been by Samuel Ruben in the U.S. Pat. No. 2,542,710 issued Feb. 20, 1951. In that patent, Ruben recognized that a steel or nickel substrate on the cathode can of a button cell tends to decrease the stability of the divalent silver oxide which is used in the cathode; and Ruben provided a silver plating or cladding on the cathode can to offset the tendency to decreased stability.
Dawson, in U.S. Pat. No. 3,484,294, issued Dec. 16, 1969, discloses a cell having a cathode which consists principally of divalent silver oxide over which a masking layer of monovalent silver oxide is placed, and above which a cellophane barrier is located between the monovalent silver oxide layer and an electrolyte absorbent layer. The Dawson cell, however, requires that the secondary active material layer--the monovalent silver oxide layer--be electrolyte impermeable; and it is difficult to produce such a cell in commercial quantities. In addition, the cell is subject to dimensional instability due to internal gassing--i.e., oxygen evolution--at greater rates than the recombinant and/or out-gassing rates.
Davies, in U.S. Pat. No. 3,853,262 issued Dec. 10, 1974, discloses a gold additive which is incorporated in the cathode or the cathode compartment of a divalent silver oxide cell. The gold additive is said to improve the stability of the cell, but has the disadvantage of increasing the cost of the cell.
Tvarusko, in U.S. Pat. No. 3,650,832, issued Mar. 21, 1972, proposes the addition of mercury, selenium and tellurium to a divalent silver oxide cell cathode, to improve the stability and/or the electrical conductivity thereof. The method of incorporating the additives is by physical admixture or by chemical coprecipitation, during the preparation of the divalent silver oxide.
Dirkse, in U.S. Pat. No. 3,348,973, issued Oct. 24, 1967, discloses a secondary battery where there is present in said cell an additive having the general formula of tridecyloxypoly (ethylenoxy) ethanol said additive having been incorporated in the electrolyte or in the zinc anode, preferably in the zinc anode. Dirkse states that the life of the cell is significantly prolonged by the use of the additive.
Ruetschi et al, in U.S. Pat. No. 3,057,944, issued Oct. 9, 1962, discloses a silver oxide cathode in a primary or secondary system where a surface active heteropolar substance is admixed either to the electrolyte or the silver cathode in an amount ranging from about 0.001 gram to about 0.2 gram per gram of silver; whereby there may be formed a polar chemical group which can attach itself to the surface of the silver oxide particles of the cathode, with the remainder of the molecule being hydrophobic in nature so as to hinder contact with the electrolyte. Excessive gassing of the divalent silver oxide when wetted by an alkaline electrolyte is reduced.
In all of the above prior art cells, as in the present invention, the anode or negative electrode is generally comprised of an amalgamated zinc powder; and the alkaline electrolyte is an aqueous solution of potassium hydroxide, sodium hydroxide, or a mixture thereof. Additional additives may also be included in the anode or electrolyte.